The SIMD1 was enhanced in the Solar Regulator version which
supplies a constant 2V after triggering and permits driving
LEDs
without using current limiting resistors with constant
brightness. Note that you can use this Low Drop Out (LDO)
linear regulator for various applications but like all
linear regulators it is "lossy". This is not a problem for
controlling LEDs
since they would otherwise use "lossy" resistors
but for motors it is a different story.

Try this new SIMD1 / Solar Voltage Regulator for use with
blinking LED
circuits (pummers).
It turns on when it gets dark, just like a D1
but the output voltage is regulated to about 2V (depending
on the reference LED Vf).

The SIMD1 / solar regulator circuit draws no current
during charging and when turned on, it draws less than 100uA
with a maximum 10ma output current.
The regulator provides constant LED
brightness during the discharge and turns off when 1F solar
capacitor
voltage drops below the LED
turn on voltage. The LED
used for voltage reference in the solar regulator feedback
loop and the LEDs
used for the flasher must be the same high efficiency type
to match the forward voltage specs. This circuit
is ideal for supplying voltage to a Bicore or 74HC14
LED
flasher since it eliminates the LED
current limiting resistors and greatly reduces current
consumption of the HC flasher circuits.

One interesting alternative would be to substitute a 5V
NiCad (4 cell) battery
for the 1F supercap which acts to increase the storage
capacity many fold for use with flag waver motors,
pendulums, etc. With higher load current,
the 100K resistor
may be replaced with 20K for up to 50 ma output current.
The quiescent current
of the regulator remains very low and is proportional to the
load current
for high efficiency operation during discharge.

CHARGING
The solar cell charges a 1F capacitor through a 1N34A
Germanium
diode to a maximum voltage of 5.5V. While the charging
current
flows through the diode
the voltage at the cathode
(stripe) is about 100 mV negative with respect to the 0 V
line. This negative voltage is applied through a 100K
resistor
to the base of a 2N3904 NPN
transistor Q1 and holds that transistor
off. This cuts off the basecurrent
for the 2N3906 transistor
(PNP)
Q2 and the output of the regulator will be zero volts.

SWITCHING
Rapid switching is very important in this type of circuit
because a circuit
that is half on draws power, draining the capacitor,
but performs no useful work. On the SIMD1/ Solar Regulator,
the output snaps on and off.

At the end of the charging cycle, when the light on the
solar
cell decreases, the negative terminal of the solar
cell starts to become more positive than the 0V line.
The base
of the NPN
Q1 must be at about +500 mV (positive) with respect to the
emitter
which is connected to the 0 V line, before it turns on and
turns on the rest of the regulator. That usually happens in
the evening but can be simulated by cupping your hand in
front of the solar
cell.

When Q1 turns on, the PNPtransistor
2N3906 - Q2 receives basecurrent
and it starts to turn on. The regulator output voltage at
the collector of Q2 increases to about + 2V when the red
LED
starts to turn on and to supply current to the base of
NPNtransistor
2N3904 - Q3. When Q3 turns on it "robs" basecurrent
from Q2. This in turn controls the basecurrent
for Q2 and the regulator output will stabilize at +2 V. The
10K resistor from the regulator output to the negative
terminal of the solar
cell provides positive feedback to the regulator turn on
by loading the solar
cell down so that it's output voltage drops even more
and the regulator "snaps" on. Note that the red LED
is used for reference voltage only and does not actually
light up.

DISCHARGING
The output voltage at the collector
of Q2 remains at 2V while the voltage on the main capacitor
can range from a full charge at 5.5V to 2.1V at the end of
the discharge cycle. If no load is attached to the regulator
the capacitor
voltage will drop very slowly because of leakage and a small
amount of current
required for the active regulator (<50 uA). When a HC
chip like a 74HC240
or 74HC14
is powered from the 2V regulated output, the current
for that chip is also very low.

If the HC chip has a LED
connected to the output which the same type of LED
that is used for reference, then the current
will be limited by a small voltage drop on the HC driver
output. Since the regulated voltage is constant the
brightness of the LED
is also constant.

When the voltage on the 1F cap drops below 2V, the
regulator reference LED
turns off and the base currents
of Q1 and Q2 increase discharging the remaining charge on
the cap and turning any attached circuit rapidly off. At
some point the voltage of the solar cell even in dim light
is higher than the remaining charge on the capacitor
and if there is sufficient light (usually in the morning)
the charging cycle repeats all over again. If the Sun is
bright and the solar
cell was shielded by your hand, then exposing the
solar
cell to the bright sunlight generates enough power to
turn the regulator off and force the circuit
back into the charging cycle.

The PowerSaver Flasher uses capacitive output coupling to
produce brighter shorter flashes and has a much lower
average current drain than standard bicore or 74HC14
flashers. The PS Flasher with one LED circuit (2 LEDs) runs
all night from a 1F cap charged to 5.5V. Up to 12 LEDs can
be controlled with one 74HC14
flasher and probably would run for 2 hours from a full
charge. Use a range of timing resistors between 1M and 4.7M
for each oscillator to give a random light show
appearance.